The inventors of the present disclosure have recognized that emissions produced by an engine shortly after start-up can constitute a large portion of the entire drive cycle emissions. Engines utilizing direct in-cylinder injectors can experience even greater emissions due to reduced fuel vaporization and atomization under certain conditions. For example, after start-up of the engine, the fuel system pressure may not yet have attained a pressure to cause sufficient atomization of the fuel within the combustion chamber, which may result in increased emissions. Furthermore, lower engine temperatures after start-up can further reduce the vaporization rate of the directly injected fuel. As yet another example, vaporization rate can vary with fuel composition. For example, blended fuels containing gasoline and alcohol may have lower vaporization rates than fuels containing only gasoline or lower concentrations of alcohol.
As one approach, at least some of the above issues may be addressed by a method, whereby in a cylinder of a directly injected internal combustion engine, the method comprises: closing an exhaust valve of the cylinder; opening an intake valve after the exhaust valve has closed; moving a piston of the cylinder away from top dead center between the closing of the exhaust valve and the opening of the intake valve to expand the cylinder volume and reduce a pressure within the cylinder to below a pressure of an air intake manifold of the cylinder; and initiating an injection of fuel directly into the cylinder via a direct in-cylinder injector after the exhaust valve has closed. In some examples, the inventors have identified that vaporization of the fuel may be further improved if the fuel injection is initiated at substantially the same timing as the intake valve opening or after the intake valve opening.
In this way, fuel vaporization may be improved by coordinating the timing of the directly injected fuel with intake valve opening while also utilizing negative valve overlap between the intake and exhaust valves to generate reduced pressures within the cylinder. These reduced cylinder pressures can improve the vaporization rate of the directly injected fuel since the boiling temperature of the fuel is reduced at lower cylinder pressures. Furthermore, the rate of air entrainment into the cylinder at intake valve opening can be increased by reducing the pressure within the cylinder to below the pressure of the intake manifold, thereby improving mixing of the entrained air and with the directly injected fuel which can further increase the fuel vaporization rate. Thus, emissions produced after engine start-up can be reduced by improving fuel vaporization as described herein.